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Larger enums to represent and discriminate. Null is just syntax sugar for the one-kind of absence case, and multiple-null-likes eliminates all benefits of that syntax sugar, so you might as well revert to a real discriminator.

Ideally you can force the use of the discriminator… but that depends on your type system

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The problem with flattening the two absence cases into an enum is composability and generality because it forces the "outer null" case to intrude into concrete types. Say you have a key value mapping Map<String, T> and you represent changes to this map as Map<String, Option<T>>. One day T itself is Option<Int> so you end up Map<String, Option<Option<Int>>. If you want to use e.g. Option2 for that latter case you lose generality.

Where the "double absence" issue comes up in practice, it's usually in a context where it does make sense to represent and handle the first type of absence separately from the second type.

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